1. Field of the Invention
It is a generally known fact that silver "tarnishes" even when it is not in use. It is only a matter of time before it develops dark, brownish, bluish to blue-black stains or completely discolors and, hence, is said in common usage to have "tarnished".
In practice, the machine washing of table silver also involves recurring problems in the form of tarnishing and discoloration of the silver surfaces. In this case, silver can react to sulfur-containing substances which are dissolved or dispersed in the wash liquor because, in domestic dishwashing machines (DDWM), food residues, including mustard, peas, egg, and other sulfur-containing compounds, such as mercaptoamino acid, are introduced into the wash liquor. The much higher temperatures prevailing in dishwashing machines and the longer contact times with the sulfur-containing food residues promote the tarnishing of silver by comparison with manual dishwashing. In addition, through the intensive cleaning process in dishwashing machines, the silver surface is completely degreased and, hence, becomes more sensitive to chemical influences.
Where detergents containing active chlorine are used, tarnishing by sulfur-containing compounds can largely be prevented because these compounds are reacted to sulfones or sulfates by oxidation of the sulfidic functions in a secondary reaction.
However, the problem of tarnishing in the case of silver became topical again when active oxygen compounds, such as sodium perborate or sodium percarbonate for example, were used as an alternative to active chlorine compounds to eliminate bleachable soils, for example tea stains/tea coatings, coffee residues, dyes from vegetables, lipstick residues and the like.
These active oxygen compounds are used in conjunction with bleach activators above all in modern low-alkali machine dishwashing detergents of the new generation. These modern detergents generally consist of the following functional components: builder component (complexing agent/dispersant), alkali carrier, bleaching system (bleaching agent+bleach activator), enzymes and wetting agents (surfactants).
Basically, the silver surfaces react more sensitively to the modified formulation parameters of the new-generation detergents free from active chlorine with their reduced pH values and activated oxygen bleaching. During the machine dishwashing process, these detergents release the actual bleaching agent, hydrogen peroxide or active oxygen, in the wash cycle. The bleaching effect of detergents containing active oxygen is enhanced by bleach activators so that a good bleaching effect is obtained even at low temperatures. In the presence of these bleach activators, peracetic acid is formed as a reactive intermediate compound. Under the modified washing conditions, not only are sulfidic coatings formed in the presence of silver, oxidic coatings are also formed on the silver surfaces through the oxidizing effect of the intermediately formed peroxides or the active oxygen. Chloride coatings can also be formed in the presence of high salt concentrations. In addition, tarnishing of the silver is intensified by relatively high residual water hardness values during the wash cycle.
Avoiding the corrosion of silver, i.e. the formation of sulfidic, oxidic or chloridic coatings on silver, is the subject of numerous publications. In these publications, the corrosion of silver is prevented above all by so-called silver protectives.
2. Discussion of Related Art
GB 1,131,738 describes alkaline dishwashing detergents containing benzotriazoles as corrosion inhibitors for silver. U.S. Pat. No. 3,549,539 describes highly alkaline machine dishwashing detergents which may contain inter alia perborate as oxidizing agent in conjunction with an organic bleach activator. Additions of inter alia benzotriazole and iron(III) chloride are recommended to prevent tarnishing. pH values of, preferably, 7 to 11.5 are mentioned. EP 135 226 and EP 135 227 describe low-alkali machine dishwashing detergents containing peroxy compounds and activators in which inter alia benzotriazoles and fatty acids may be present as silver protectives. Finally, it is known from DE-OS 41 28 672 that peroxy compounds activated by addition of known organic bleach activators prevent the tarnishing of silver in highly alkaline detergents.